1. Field of the Invention
This invention relates to a flux which has particular utility in submerged arc welding of steel, which enables excellent welding workability and which provides a weld metal of excellent mechanical properties.
2. Description of the Prior Art
Submerged arc welding is widely used for butt welding of steel since it is the most economical, in terms of labor costs, for such welding procedures. In recent years there has been significant improvements made in submerged arc welding processes, which have made that technique even more efficient. For instance, in recent years, such innovations as one side submerged arc welding, multielectrode welding and large heat input welding for double groove joints, have been developed. However, in submerged arc welding processes which require large heat input, the degree of dilution of the base metal is often so large that the weld metal can be adversely affected by the chemical composition of the base metal, and a columnar structure can develop in the weld metal. This tends to coarsen the crystal grains of the weld metal and cause the formation of non-metallic inclusions in the coarse grain boundaries. Accordingly, it has been very difficult to obtain a weld metal of sufficiently good impact properties, as might have been expected.
Multielectrode welding which requires large heat inputs not only adversely affects the impact properties of the weld metal, but it also can be a detriment to the welding operability. It has been attempted to overcome these defects using conventional fluxes, however, the use of such conventional fluxes have not proven to be entirely successful. Other methods attempted to alleviate these difficulties included decreasing the oxygen content in the weld metal by increasing the basicity of the flux; refining the microstructure of the weld metal by the addition of Mo into the filler wire or into the flux; refining the microstructure of the weld metal by addition of B into the flux or the wire in the form of simple substance or ferroboron, in vey small amounts. These methods likewise have not proven to be completely satisfactory for all purposes.
If the basicity of the flux is increased so as to decrease the oxygen content of the weld metal, it is impossible to maintain a constant chemical composition for the weld metal, and it is also impossible to uniformly obtain stable impact properties in the weld metal.
If an alloying element such as Mo is added to the flux or to the weld wire, the tensile strength of the weld metal will be unnecessarily increased, causing an increase in crack sensitivity.
If B is added into the flux or into the welding wire in the form of elemental boron or a ferroboron, the impact properties of the weld metal will be greatly improved, however, it will be difficult to prevent segregation of B during welding especially if the welding conditions are varied by a large heat input or during production of the flux or the welding wire. Moreover, if the welding groove is not even, it is critical and difficult to control the welding conditions to obtain an even weld bead. Accordingly, satisfactory effects have not heretofore been obtainable using conventional fluxes, when large heat input welding processes are used.